zcow

Manages temporary zram backed COW (Copy On Write) devices. zcow uses Device Mapper to combine a zram device and the origin device, such that unchanged blocks are read from the origin device and written blocks are stored in the zram device.

Dependencies

• Linux
• bash
• udevadm
• dmsetup
• zramctl

Synopsis

Create a zram device for /dev/sda4:

$ zcow create /dev/sda4

Create a zcow device for /dev/sda4, but with the custom name foo:

$ zcow create /dev/sda4 foo

List all zcows:

$ zcow
sda4	/dev/sda4	/dev/zram0
foo	/dev/sda4	/dev/zram1

Get info about the zcow named foo:

$ zcow info foo
Origin:	/dev/sda4
COW:	/dev/zram1

NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram1 lzo-rle       100M   0B    0B    0B       8 

Remove the zcow named foo:

$ zcow destroy foo

systemd

Register a systemd unit that manages a zcow device for /dev/sda4:

$ sudo systemctl enable zcow@/dev/sda4

Install

wget -O zcow-0.1.0.tar.gz https://github.com/postmodern/zcow/archive/v0.1.0.tar.gz
tar -xzvf zcow-0.1.0.tar.gz
cd zcow-0.1.0/
sudo make install

PGP

All releases are PGP signed for security. Instructions on how to import my PGP key can be found on my blog. To verify that a release was not tampered with:

wget https://raw.github.com/postmodern/ruby-install/master/pkg/zcow-0.1.0.tar.gz.asc
gpg --verify zcow-0.1.0.tar.gz.asc zcow-0.1.0.tar.gz

Limitations

• Cannot work with loopback devices.

• Cannot mount zcow devices for some unknown reason:

  mount: /mnt: wrong fs type, bad option, bad superblock on /dev/mapper/zcow-test, missing codepage or helper program, or other error.